This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to 9927219.7 filed in Great Britain on Nov. 17, 1999; the entire content of which is hereby incorporated by reference.
The present invention relates to telecommunications networks and in particular to cellular mobile telecommunications networks.
FIG. 1 of the accompanying drawings illustrates a cellular mobile telecommunications network which defines a number of cells C1 . . . C8, each of which has a basestation BS1 . . . BS8. Mobile stations MS1 . . . MS3 are able to roam through the cells. In the simplest case, a mobile station will communicate with the basestation of the cell in which it is located. When more than one mobile station wishes to communicate with a basestation, the telecommunications system must allocate available radio frequency spectrum to the various mobile stations.
The amount of radio frequency spectrum is a finite resource and many cellular operators do not have as large an allocation as they would wish. There are two basic approaches for allocating channels to mobile stations to give high capacity. One approach is to use a high number of small cells, and the other is to provide a lower number of higher capacity cells. The former approach tends to be more costly to implement than the latter because of the fixed costs associated with the provision of a large number of cells.
The present very high (and still increasing) penetration of mobile phone usage therefore makes increasing the efficient use of radio frequency spectrum a critical and commercial factor for both cellular radio system manufacturers and operators. Higher spectral efficiency allows denser frequency reuse patterns, and thus allows a frequency limited operator to deploy larger (and thus fewer) cells.
To some extent, all radio channels in a cellular system interfere with each other, both in the frequency and time domains. It is therefore desirable to attempt to allocate channels such that interference between channels is at least predictable.
This can be realised by trying to allocate radio channels which have relatively low levels of interference from existing connection allocations and conversely which will not present unacceptable levels of interference to such existing connections that were allocated based on an interference situation predating the current allocation.
An efficient allocation scheme is one where the carrier/interference ratio is approximately equal for the radio links towards all active mobile stations, said carrier to interference ratio being as small as practicable compatible with the demand that the signal shall be carried with acceptable fidelity. Since different mobiles are located in different physical places that have correspondingly different propagation characteristics, these radio links typically have corresponding different values for signal strength.
The problem of channel allocation is to find an algorithm that gives this mapping of connections onto spectrum such that all parts of the available spectrum have as equal as possible carrier to interference ratio. Specifically this means that no part of the available spectrum shall be (globally) unused while other parts carry multiple connections that mutually interfere: rather the used channels shall be spread out as evenly as possible in the time/frequency space, in which the mean variance of carrier/interference (C/I) ratios are minimised.
A well known and understood multiple access scheme is the Time Division Multiple Access (TDMA) scheme in which mobile station channels are distributed in time. A brief description of a known TDMA system is given below, and more detailed information can be found, for example, in xe2x80x9cThe GSM system for mobile communicationsxe2x80x9d by Mouly and Pautet, ISBN 2-9507190-0-7.
In this description, channel allocation is described in the context of a TDMA cellular system constructed according to the GSM recommendations, since this constitutes the most widely used cellular standard at this time. However one skilled in the art will readily appreciate that the principles described can equally be applied to cellular systems built to other TDMA standards.
In the TDMA system defined in the GSM recommendations, signals are transmitted in a sequence of frames, each of which comprises 8 timeslots. A frame is illustrated in FIG. 2. The timeslots are conventionally numbered 0 to 7. Each timeslot is a window in time that may be used to carry a burst of radio energy as a layer 1 carrier for one basic physical channel (BPC).
It is possible (and in fact is normally the case) that a cell will have several transceivers and thus can support several frames concurrently. Two frames are illustrated in FIG. 3. In a GSM system, concurrent frames within one cell must be synchronised with one another. However, it is normally the case that frames in different cells are not synchronised.
When a new connection is established in a cellular radio system (and in certain other circumstances) the system must select a channel (BPC) from those which are currently available. This function is called xe2x80x9cchannel allocationxe2x80x9d.
In an efficient system, the channel allocation method may be complex, since the objective is that connections will always have undisturbed radio links, and each radio link allocated presents interference towards other channels in other cells. The method may consider many factors in its attempt to solve for a mapping of all currently used radio links on the available spectrum that gives the best overall interference level on each individual channel.
Typically there are many candidate channels available from which one must be selected to be allocated to the requested new connection.
Many strategies are used and/or have been proposed and are known to those skilled in the art. Some deterministically select channels based on the characteristics of the mobile station involved, e.g. its propagation conditions, distance from the base station, if it is moving etc. These mechanisms typically reduce the number of xe2x80x9cfavouredxe2x80x9d candidates, but typically still do not always result in just one remaining candidate channel.
Typically there are many candidate channels available from which one must be selected to be allocated to the new connection in question.
Current systems monitor candidate (ie currently idle) channels to determine the instantaneous amount of interference present on each, and thereby select the channel with the least interference for allocation.
This xe2x80x9cidle channel measurement based allocationxe2x80x9d scheme has several deficiencies. Each channel in a given cell potentially suffers interference from channels in surrounding cells which use the same frequencies at the same time.
These mutually interfering connections are established, handed over and terminated independently. The result is that over period of minutes (the duration of a typical call made with a mobile phone), the interference levels on each channel will repeatedly change as new calls are set up and old calls end.
If power control is in use, there will be further variations in the interference levels found on the different channels.
The result is that a channel which appears to be the least disturbed at the instant when it is selected and allocated typically will not remain the least disturbed channel in the cell until the end of the connection.
During busy periods in the capacity scenario, the interference distribution across the candidate channels will change almost continuously, that is to say, will be unstable. Channels allocated based on the instantaneous quality at allocation time will not maintain any systematic advantage over the mean channel quality over any useful time period.
In non-synchronous TDMA networks, it is statistically improbable that one timeslot in one cell interferes neatly with only one timeslot in surrounding cells. This is due primarily to the different cells involved not being synchronised, but can be further complicated by propagation delays in some scenarios, as is well understood.
This means that one channel allocated in one cell typically interferes partly with more than one channel in the interfering cell. Such a scenario is illustrated in FIG. 4, in which a channel 2 of a first cell overlaps the boundary of two channels 4 and 6 of a second cell. This overlap is illustrated by the shadow 3 of the channel 2.
The same effect occurs with each active channel within a cell, resulting in the situation that a minority of channels active in each individual cell may (statistically) result in a majority of channels in surrounding cells being at least partly disturbed. For example, four active channels allocated in one cell can result in all bursts in the second cell being disturbed.
FIG. 5 illustrates a scenario in which 4 allocated timeslots 8 alternate with 4 idle timeslots 10 in the first cell. The second cell also includes alternating allocated 12 and idle 14 timeslots. If the cells were synchronised it is possible that there would be no overlap, however in the typical case shown in FIG. 5, there will be overlap and in this case all allocated timeslots find themselves partly overlapped (shown as a shadow 9 in FIG. 5).
It is therefore desirable to allocate channels in such a way that interference can be reduced.
According to one aspect of the present invention, there is provided a method of allocating timeslots in a Time Division Multiple Access (TDMA) telecommunications system comprising steps of:
defining a first timeslot for allocation to a first channel;
allocating the first timeslot to the first channel to produce a first allocated timeslot;
allocating a second timeslot to a second channel to produce a second allocated timeslot, such that the second allocated time slot is adjacent in time to the first allocated timeslot;
allocating subsequent channels to respective further timeslots, to produce further allocated timeslots, such that each further allocated timeslot is adjacent in time to a previously allocated timeslot, thereby forming a contiguous group in the time domain.
According to another aspect of the present invention, there is provided a method of channel allocation in a Time Division Multiple Access (TDMA) telecommunications system in which timeslots are allocated to channels such that allocated timeslots form a group which is contiguous in the time domain.
According to another aspect of the present invention, there is provided a method of allocating channels in a TDMA cellular radio system, such that the channels allocated in any one cell are clustered in the time domain which is stable over the long term and further that this centre of the cluster is configured such that it advantageously falls in a region of the time domain that over the long term has below average interference resulting in a mean improvement in the interference level experienced.